1. Field of the Invention
This invention relates,to an image forming apparatus and, more particularly, to an image forming apparatus of the electrophotographic type wherein a toner image formed on a photosensitive member is transferred to a recording medium to form an image.
2. Description of the Prior Art
In an image forming apparatus of the type mentioned, an image is formed as an electrostatic latent image on a charged photosensitive member, and the latent image is developed with toner into a visible toner image. Then, the visible toner image is transferred to a recording medium and then processed by the step of heating fixing processing to obtain a complete image. Various methods are available to transfer a toner image to a recording medium, and one of the methods employs a transfer roller.
Transfer using a transfer roller is performed such that, when a recording medium is passed between a photosensitive member, on which a toner image is formed, and the transfer roller, which is disposed in contact with the photosensitive member, a transfer bias potential of the opposite polarity to that of the charge applied to the toner is applied to the transfer roller to attract and transfer the toner image on the photosensitive member to the recording medium.
In an image forming apparatus having such a construction as described just above, it is a principle to supply a recording medium of a size suitable for the size of an image to be formed. However, several sizes are available for an image formed, for example, on a laser printer, and a recording medium suitable for the size of an image cannot always be supplied. When the size of an image to be formed is smaller than that of the recording medium, no particular trouble will occur. However, in the reverse case, that is, when the toner image formed on the photosensitive member is greater than the recording medium to which the toner image is to be transferred, toner of a portion of the toner image on the photosensitive member which protrudes from the recording medium will be transferred to the transfer roller and may soil another recording medium to be supplied subsequently or substantially cause a drop of the transfer bias potential applied to the transfer roller, resulting in incomplete transfer.
Therefore, it has been proposed to apply, before and/or after printing processing for an image, a potential of a polarity reverse to that for transfer to the transfer roller to transfer toner remaining on the transfer roller to the photosensitive member to clean the transfer roller. An apparatus of the type just described is disclosed, for example, in U.S. Pat. No. 5,132,738. Another image forming apparatus has been proposed and is disclosed in Japanese Patent Laid-Open No. Hei 4-83272 wherein the size of an image to be formed, which has been designated by means of keys of an operation panel or like means, and the size of a recording medium supplied are compared with each other, and when the sizes do not coincide with each other, it is determined that a trouble has occurred and recording processing for the image is interrupted, thereby to prevent soiling caused by transfer of toner.
With the former apparatus wherein the transfer roller is cleaned before and/or after printing processing for an image, much time is required for driving of the apparatus other than for formation of images, resulting in reduction of the life of the apparatus. Further, if cleaning is performed between each two successive image printing cycles, then even if the time required for one printing cycle is equal, if it is tried to obtain a plurality of prints of the image, surplus times are required, and consequently, much time is required.
On the other hand, with the latter apparatus, when the designated image size and the size of the recording medium do not coincide with each other, it is determined that a trouble has occurred and recording of the image is interrupted, and consequently, there is a disadvantage in that it is impossible to automatically restore a recording enabled condition.
Further, with the latter apparatus, it sometimes occurs that, even if the designated image size and the size of the recording medium do not coincide with each other, an image actually formed is accommodated within the recording medium, and it is another disadvantage in that, also in such a case, recording of the image is interrupted. Further, the latter apparatus cannot satisfy the demand to print the image although it is known that the image is partly broken (principally at a peripheral portion thereof).
Further, in such a laser printer as described above, when a recording medium is fed by feeding means, incomplete feeding such as a failure to pick up a recording medium or clogging of a recording medium, that is, jamming, sometimes occurs. Depending upon a timing at which such jamming occurs, a toner image formed already on the photosensitive member may be transferred directly to the transfer roller and soil another recording medium supplied subsequently.